Electrical Properties of Doped Tin Dioxide Thin Films Deposited Using Femtosecond Pulsed Laser Ablation
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ELECTRICAL PROPERTIES OF DOPED TIN DIOXIDE THIN FILMS DEPOSITED USING FEMTOSECOND PULSED LASER ABLATION. J. E. Dominguez, L. Fu, and X. Q. Pan, The University of Michigan, Department of Materials Science and Engineering, Ann Arbor, MI, 48109 ABSTRACT Tin dioxide thin films are widely used as chemical sensors. The mechanism for gas sensitivity depends on the chemisorption characteristics of the oxide surface and on the electronic characteristics of the film. Chemical doping and the film microstructure can influence the electronic properties. In this paper tin dioxide thin films doped with rare earth and transition metals were deposited on the (10 1 2) -oriented sapphire substrates by femtosecond pulsed laser deposition. The resulting films were single crystalline with a thickness of about 15 nm and 30 nm. The response of the films to reducing gases was measured in a gas reactor at high temperature. The electrical transport properties of the films were determined by Hall effect measurements within the gas reactor. The valence and ionic radius of dopants have strong influence on the mobility and concentration of electrical carriers (i.e. electrons) and thus affect the response of the thin films to reducing gases. A model correlating the dopant characteristics to the electrical properties was developed. INTRODUCTION Tin dioxide (SnO2) is an n-type semiconductor that has a wide range of applications as transparent electrodes, solar cells, and gas sensors. Its electrical and electrochemical properties strongly depend on the microstructure and stoichiometry of the material. For gas sensors based on SnO2, both bulk dopants and surface modifiers are commonly used to improve the gas sensitivities.1 The surface modifiers such as Pd and Pt can promote the adsorption of oxygen molecules on the oxide surface.2 Doping in SnO2 can change the electrical properties and microstructure of the film. For example, it was reported that doping with elements such as In, Ca and W had an effect of limiting the grain growth of SnO2 during high temperature sintering, possibly by providing a barrier to surface diffusion.3 Dopants consisting of different valences and ionic radii will have different effects on the electrical and electrochemical properties. The sensitivity of SnO2 films doped with acceptors has been shown to be higher than that of pure materials.4 It was also reported that polycrystalline SnO2 thin films doped with Cr and Co show varistor properties.5 Reproducibility of these results is poor and little is understood about the mechanism of the effect of doping on the properties of SnO2 films, especially, the influence of the size and valence of dopants on electrical transport properties. In this paper we report our preliminary studies on these aspects. In order to avoid complications due to grain boundary effects, SnO2 single crystal films grown on the (10 1 2) α-Al2O3 (sapphire) substrates were used in this work. EXPERIMENTAL SnO2 thin films with a thickness of 15 and 30 nm were fabricated using pulsed laser deposition technique. A
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